I am guessing it's a spinner of one of the Callibaetis species with broken-off tails. The clue I'm focusing on is the glassy wing with mottled black and white markings. I know almost nothing about the different species of Callibaetis and their spinners, though, so I really don't know.

I do know that even within the same species male spinners can differ from female. So with all the callibaetis species out there, one callibaetis photo won't necessarily resemble another. That said, I did find this Callibaetis photo online, which looks somewhat similar to yours, although it's from Idaho: